If a read error is encountered during 'qemu-img compare', we
were printing the "Error while reading offset ..." message twice;
this was because our helper function was awkward, printing output
on some but not all paths.  Fix it to consistently report errors
on all paths, so that the callers do not risk a redundant message,
and update the testsuite for the improved output.

Further simplify the code by hoisting the conversion from an error
message to an exit code into the helper function, rather than
repeating that logic at all callers (yes, the helper function is
now less generic, but it's a net win in lines of code).
Signed-off-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NJohn Snow <jsnow@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>